The invention relates generally to systems for determining the position and motion of one vehicle (aircraft, spacecraft or ship) with respect to another and, more particularly, to optical systems for determining bearing, attitude, range and motion of one such vehicle relative to another at ranges on the order of kilometers but with an exceptionally high degree of accuracy at close range in order to facilitate link-up or rendezvous of the vehicles, such as for refueling or docking operations.
Refueling of aircraft in-flight requires that the refueling operation be accomplished within a certain minimum period of time since the receiving aircraft may be low on fuel and the pilots may be unduly fatigued from putting in excessively long flight time. In addition, range, motion and orientation of the aircraft relative to each other must typically be kept within certain maximal limits especially for safety reasons. Docking of two spacecraft in orbit additionally presents unique problems because orbital mechanics principles require a high degree of skill in docking spacecraft, and trial and error docking maneuvers would be costly in terms of time and propellant usage.
Docking maneuvers must be accomplished with a minimum of velocity and orientation changes to either or both spacecraft in order to avoid undue alteration of spacecraft orbit and attitude. Velocity changes imposed to bring the spacecraft together change the orbit and attitude in complex ways that can exceed manual pilot capability and make it extremely difficult to achieve docking in minimum time and with minimum propellant consumption, while satisfying docking constraints. These docking constraints are 1) assuring that effluent contamination from the attitude control thruster is kept below certain limits; 2) docking within a specified time period; 3) approaching along a specified flight path; 4) conforming to range/position and rate of change of position of target vehicle; 5) conforming to attitude and attitude rate of target vehicle; 6) acquiring and formatting data for use by navigation computers at commensurate rates and accuracies; 7) attaining and maintaining a desired degree of accuracy of target vehicle state vectors i.e., position, attitudes, rates, etc. so that the control system can meet the required impact or closure velocity, range, etc. These requirements are more stringent as the spacecraft near each other. Furthermore, docking ports are not necessarily easily observed by the target vehicle pilot, and, therefore, some means of observing and quantitatively evaluating and correcting docking range and attitude is required.
Since direct viewing of the refueling or docking operation by the pilot would not typically provide all the information required regarding vehicle position and motion with the degree of accuracy required, many prior art systems have been designed which are able to measure certain relevant position and motion parameters without requiring direct pilot observation of the refueling operation. Some prior art systems use optical systems to determine the location and range of the refueling boom and receiver aircraft relative to the tanker aircraft and to the boom. Examples of such a system are disclosed in U.S. Pat. Nos. 4,025,193 and 3,917,196 both to Pond et al. The Pond systems use an optical source on the tanker aircraft to transmit a light beam which is reflected by suitably positioned reflectors on the receiver aircraft and the boom. An optical sensor on the tanker aircraft determines azimuth, elevation and range of the receiver aircraft and the boom by focusing the reflected light onto an image dissector tube and feeds this information to a computer and to a cathode ray tube rectangular coordinate display. The range of the two aircraft is determined by comparison of the phase of the transmitted and reflected light beams. The range and relative positions are displayed as dots on a screen. A primary disadvantage with the Pond system is that determination of range by phase comparison limits range measurement to relatively short distances on the order of one-half of a wavelength.
A position and motion measuring system for in-flight vehicles is thus needed that has the capability of measuring position and motion over both close and moderate distances with a high degree of accuracy at close range. A position and motion measuring system is also needed that can provide such capabilities without utilizing moving parts that can wear, break or become misaligned because of vibration, shock or wear. In addition, a measuring system is needed that can provide such capabilities without being affected by light or radiation from extraneous sources or from extraneous reflections.